I will go to Savannakhet for the first time to get a single entry tourist visa.

Any advice about guest house in central location with wifi?

Anything else important come to mind from your experience there?

You can find some info here: Latest Reports Of Getting A Visa In The Region

The are now doing visa on the same day you apply. Apply by 11 am and get your passport back at 3:30 pm.

The T- 87 form can be used for tourist visa?

It's the only visa form on the immigration website but it says non immigrant

You don't use an immigration form at the consulate.

Here is their visa application form: Savannakhet .pdf

My Report from my Savanahkhet visa trip a little over a week ago:

http://www.thaivisa.com/forum/topic/785409-my-mostly-good-experience-at-savanakhet-dec11-to-dec12-2014-for-non-o-marriage-multi/

Assuming you stay overnight in Mukdahan, I would recommend the Huanum Hotel, it is only 400 Baht per night, excellent wi-fi, nice clean room, only drawback, if you can call it that, is they don't make your bed, empty the waste bin, or change the towels, but for the price and the excellent room, I could live with that.

The proprioters name is Tee, he speaks good English, and will tell you all you want to know about the bus, the crossing etc. The hotel is next to the Indoctrine Market and about 200 Mts from the Immigration office. I will certainly be going back there the next time I go to Savakhanet.